1. Type about:config in the URL address bar.
2. A large collection of alphabetically listed information will appear in the browser window.
3. Search for network.http.sendRefererHeader.
4. By default, this should be set at 2.
5. If it is set to 0 - which disables sending referrers - right click it, select Modify and enter 2 as the value.
6. If that doesn't work, try a value of 1